Field of Invention
The present invention relates in general to the technology of flash lamp, and more particularly to a method for controlling flash timing of an extension flash module of a mobile device.
Description of Related Art
With continuous advance in pixels and quality of digital photography, it has become a trend for mobile devices, such as mobile phones and tablets, to carry the function of taking photos. However, these built-in digital cameras have not been able to perform so well as conventional digital cameras under the circumstances of low lighting or backlighting.
Although some built-in digital cameras also carry a light-emitting diode (LED) supplement lamp, both the battery capacity of mobile devices and the heat dissipation issue of LEDs seriously confine the volume of fill light provided by the LED supplement lamp. When the distance between the object being photographed and the LED supplement lamp exceeds one meter, the LED fill light cannot provide adequate light source to allow pixels of a photo sensor to be properly exposed.
Xenon high-intensity discharge lamps (HIDs) can provide a large amount of supplementary lighting within a short period of time. Therefore, conventional digital cameras usually carry a xenon HID. A charger in a xenon HID converts low-voltage battery power supply into high-voltage power supply and stores it in a high-capacitance high-voltage capacitor. Operating in coordination with a mechanical shutter, the xenon HID is then triggered at a proper timing to convert the electricity stored in the high-voltage capacitor into high-brightness supplementary lighting within a very short period of time so that the pixels of a photo sensor are properly exposed under circumstances of low lighting or backlighting. A xenon HID requires a high-voltage capacitor having a capacitance from dozens to hundreds of μF and able to withstand 300 to 400 volts. In pursuit of lighter, thinner and more compact mobile devices, the very large volume of such a high-voltage capacitor fails to meet the requirements of current mobile devices. Therefore, in order not to increase the volume and weight of existing mobile devices, extension HID flash modules become a feasible and even necessary option.
According to the specifications of the capacitance in high-voltage capacitors and HID lamp tubes, the flash time of HID flash modules lasts from dozens to hundreds of microseconds (μs). How to flash at the right timing so that all pixels in a photo sensor are evenly exposed is an important issue to be solved for extension HID flash modules to become a feasible option. Mobile devices which carry a photo-taking device normally adopt a complementary metal-oxide semiconductor (CMOS) photo sensor and a rolling shutter instead of a mechanical shutter, as shown in FIG. 1. FIG. 1 is a schematic drawing of a rolling shutter in the conventional art. In FIG. 1, every line represents the time during which a row in the photo sensor performs light-sensing operation. Although the length of exposure for every photo sensor row in a frame is the same, there is a delay between the time when a photo sensor row begins or ends exposure and the time when its preceding photo sensor row begins or ends exposure. During the delay time, the photo-taking device reads the exposure data in the photo sensor row and resets the pixels so as to prepare for the exposure in the next frame. Since photos are taken in different environments and photo-taking devices are also set differently, the exposure time of one photo sensor row is in a wide range, lasting approximately from a few milliseconds to hundreds of milliseconds.
However, in coordination with a rolling shutter, an extension HID flash module usually cannot locate the optimal flash timing and fails to improve the quality of photos.